Multicellular animals are derived from a clone of cells descended from a single original cell, the fertilized egg. Embryogenesis involves the division and differentiation of multi-potential cells, each cell having the ability to develop into multiple cellular lineages. Phenotypically, the cells of such lineages can vary substantially, such as blood cells, muscle cells and neural cells, being specialized.
A wide spectrum of diseases may be treated based upon both the possession of a population of cells having multi-lineage potential and an understanding of the mechanisms that regulate embryonic cell development. For example, the capacity to generate a new population of hematopoietic cells is the basis of bone marrow transplantation, which is currently used as a treatment for a growing number of diseases including anemia, leukemia and breast cancer. In addition, transplantation of genetically altered multi-potential cells has been considered as potential therapy for a variety of different diseases including AIDS.
One of the major barriers to both the treatment of diseases and the study of the process by which an undifferentiated embryonic cell becomes committed to a particular developmental pathway is the lack of access to populations of embryonic cells that are sufficiently multi-potent to be able to develop into various lineages. In particular, much attention has been paid to the use of bone marrow stem cells as a source of multi-potential cells for therapy and experimental use. Bone marrow stem cells, however, have limited use because such populations of cells comprise a subpopulation of complex hematopoietic tissue and, therefore are rare. In addition, bone marrow stem cells have not been grown as a substantially homogeneous population in tissue culture.
Following fertilization, an egg divides over a period of days to form a blastocyst. A blastocyst includes a hollow ball of cells having an inner cell mass and a fluid-filled cavity, both encapsulated by a layer of trophoblast cells. The blastocyst then implants into the uterine wall and enters into the embryonic stage of development characterized by the formation of the placenta, the development of major internal organs and the appearance of major external body structure.
Cells from the inner cell mass of an embryo (i.e., blastocyst) can be used to derive a cell line capable of being maintained in tissue culture that is referred to as embryonic stem (ES) cells. The use of ES cells to obtain hematopoietic populations of differentiated cells has been suggested in Burkett et al., pp. 698-708. 1991, New Biologist, Vol. 3; Schmitt et al., pp. 728-740, 1991, Genes and Development, Vol. 5; Gutierrez-Ramos et al., pp. 9171-9175, 1992, Vol. 89; Keller et al., pp. 473-486, Mol. Cell. Biol., Vol 13; and Breier et al., pp. 521-532, 1992, Development, Vol. 114. The use of ES cells to obtain endothelial populations of differentiated cells has been suggested by Wang et al., pp. 303-316, 1992, Development, Vol 114. Prior investigators, however, have failed to obtain populations of totipotent cells (i.e., cells that can develop into any lineage, discussed in detail below) and pluripotent cells (i.e., cells, that while unable to develop into all lineages of cells, are at least able to develop into all hematopoietic lineages, also discussed in detail below). A reason for this failure is that the ES cells were cultured under conditions in which the cells committed to a cellular lineage early in the tissue culture process. As a result, prior investigators failed to recognize a method for obtaining substantially homogeneous populations of totipotent or pluripotent embryonic cells that are useful for therapeutic or experimental use. In addition, prior investigators failed to recognize a method for inducing substantially homogeneous populations of totipotent or pluripotent embryonic cells to develop into preferred cell types.
Thus, there remains a need to develop a population of embryonic cells that are totipotent or pluripotent and, therefore, are capable of developing into a wide variety of cellular lineages.